For mammalian cells to survive, they must be able to respond rapidly to changes in their environment. Furthermore, for cells to reproduce and carry out other cooperative functions, they must be able to communicate efficiently with each other.
Cells most frequently adapt to their environment and communicate with one another by means of chemical signals. An important feature of these signaling mechanisms is that in almost all cases a cell is able to detect a chemical signal without it being necessary for the chemical messenger itself to enter the cell. This permits the cell to maintain the homeostasis of its internal environment, thereby permitting the cell to respond to its external environment without being adversely affected by it.
These sensing functions are carried out by a variety of receptors, which are dispersed on the outer surface of the cell and function as “molecular antennae.” These receptors detect an incoming messenger and activate a signal pathway that ultimately regulates a cellular process such as secretion, contraction, metabolism or growth.
In the cell's cellular plasma membrane, transduction mechanisms translate external signals into internal signals, which are then carried throughout the interior of the cell by chemicals known as “second messengers.”
In molecular terms, the process depends on a series of proteins within the cellular plasma membrane, each of which transmits information by inducing a conformational change in the protein next in line. At some point, the information is assigned to small molecules or even to ions within the cell's cytoplasm, which serve as the above-mentioned second messengers. The diffusion of the second messengers enables a signal to propagate rapidly throughout the cell.
Abnormal cell signaling has been associated with cancer diseases. Cell signaling plays a crucial role in cell growth, proliferation and differentiation. Thus, when normal cell signaling pathways are altered, uncontrolled cell growth, proliferation and/or differentiation can take place, leading to the formation and propagation of cancer.
Cancer is the leading cause of death, second only to heart disease in both men and women. Breast cancer is the most common tumor in women, representing 32% of all new cancer cases and causing 18% of cancer-related deaths of women in the United States. In the fight against cancer, numerous techniques have been developed and are the subject of current research to understand the nature and cause of the disease, and to provide techniques for the control or cure thereof
One promising avenue for the development of cancer treatments is based on blocking abnormal cell signaling pathways. Particular efforts are directed to the elucidation and regulation of the activity of receptor and trans-membrane proteins.
The human epidermal growth factor (EGF) is a six kilodalton (kDa), 53 amino acid, single-chain polypeptide which exerts its biological effect by binding to a specific 170 kDa cell membrane receptor (EGF-Rc). The human EGF-Rc consists of an extracellular domain with a high cysteine content and N-linked glycosylation, a single transmembrane domain, and a cytoplasmic domain with tyrosine kinase activity.
Many types of cancer display enhanced EGF-Rc expression on their cell surface membranes. Enhanced expression of the EGF-Rc can increase signalling via receptor-mediator pathways which lead to pleiotropic biological effects including excessive proliferation and metastasis. Examples include prostate cancer, breast cancer, lung cancer, head and neck cancer, bladder cancer, melanoma, and brain tumors.
In breast cancer, expression of the EGF-Rc is a significant and independent indicator for recurrence and poor relapse-free survival. The epidermal growth factor receptor (EGF-Rc) of cancer cells therefore represents a potential target for biotherapy.
EGFR and its physiologic ligands, epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha), play a prominent role in the growth regulation of many normal and malignant cell types. One role the EGF receptor system may play in the oncogenic growth of cells is through autocrine-stimulated growth. Cells which express EGFR and secrete EGF and/or TGFalpha can stimulate their own growth, thereby creating a cancerous condition.
An autocrine growth stimulatory pathway analogous with that proposed for epidermal growth factor receptor and its ligands may also be employed by a growing list of oncogene encoded transmembrane proteins that have a structure reminiscent of that of the growth factor receptors.
The HER-2/neu or c-erbB-2 oncogene belongs to the erbB-like oncogene group, and is related to, but distinct from EGFR. The ErbB-2 gene encodes a 185 kD transmembrane glycoprotein that has partial homology with other members of the EGFR family. The expressed protein has been suggested to be a growth factor receptor due to its structural homology with EGFR. However, known EGFR ligands, such as EGF or TGF. alpha do not bind to p185-erbB-2.
The erbB-2 oncogene has been demonstrated to be implicated in a number of human adenocarcinomas leading to elevated levels of expression of the p185 protein product. For example, the erbB-2 oncogene has been found to be amplified in breast, ovarian, gastric and even lung adenocarcinomas. Furthermore, the amplification of the c-erbB-2 oncogene has been found in many cases to be a significant, if not the most significant, predictor of both overall survival time and time to relapse in patients suffering from such forms of cancer. Carcinoma of the breast and ovary account for approximately one-third of all cancers occurring in women and together are responsible for approximately one-fourth of cancer-related deaths in females.
Significantly, the c-erbB-2 oncogene has been found to be amplified in 25 to 30% of human primary breast cancers and it has been associated with a high risk of relapse and death. In breast cancers with erbB-2 overexpression abnormal cell proliferation is believed to be caused by extremely high tyrosine kinase activity and the resulting high level of signal transduction.
Overexpression of HER-2 has also been found to be associated with increased resistance to chemotherapy or patients with elevated levels of HER-2 respond poorly to many drugs. It is believed that decreasing the levels of HER-2 will allow chemotherapeutic drugs to be more effective. Therefore, therapies targeted at erbB-2 have the great therapeutic potential for the treatment of breast cancers.
In view of the above, the development of new and potent anti-breast cancer drugs and the design of treatment protocols directed at the regulation of erbB-2 activity is an exceptional focal point for research in the modem therapy of breast cancer. Drug targeting is a particularly attractive approach for killing malignant cells, when leaving normal tissue unharmed is achieved.
ErbB-2 is a clinically proven therapeutic target for breast cancer. Indeed, the recently completed phase M clinical trial of anti-Her2 Herceptin provide evidence that systemic administration of Herceptin, alone and in combination with cytotoxic chemotherapy in patients with erbB-2 overexpressing prinary tumors, can increase the time to recurrence and overall response rates in metastatic breast cancer. Herceptin is recognized as the first in what promises to-be a wave of therapies attacking cancer at its genetic roots.
Certain limitations are associated with large molecule strategies, including poor delivery, poor in vivo stability, possible immune response and high cost. Accordingly, it is highly desirable to provide therapies based on small molecules targeted at interfering with erbB receptor-mediated signal transduction pathways (including erbB-2, erbB-3 and erbB4). Compared to therapies based on large drug molecules, such as therapeutic antibodies, small molecule drug therapies have a number of advantages, including good oral availability and low cost.
A number of criteria should be considered in the development of small molecule erbB-2 kinase inhibitors, including good potency, selectivity, cell permeability, bioavailability, appropriate pharmacokinetics and non-toxicity.
In breast cancers with erbB-2 overexpression, abnormal cell proliferation is caused by the extremely high tyrosine kinase activity and resulting high level of signal transduction. Drugs blocking this extremely high erbB-2 tyrosine kinase activity could have the potential to shut down signaling pathways mediated by erbB-2. Thus, erbB-2 kinase inhibitors that are capable of entering the cell, blocking tyrosine kinase activity and shutting down the signal transduction pathway mediated by erbB-2 may be used as potential therapeutic agents for the treatment of breast cancer. Furthermore, it has been shown that tyrosine kinase inhibitors synergize with antibodies to EGFR to inhibit the growth of aquamous cell carcinoma in vivo. Thus, a specific erbB-2 kinase inhibitor may also have synergistic effects with Herceptin in the treatment of breast cancer.
The development of small molecule kinase inhibitors of the EGFR family of receptors tyrosine kinases has been so far focused on EGFR itself. Very potent and selective EGFR small molecule kinase inhibitors have been reported and some EGFR small molecule kinase inhibitors have advanced to phase I/II clinical trials for the treatment of certain cancer forms. To date, very few kinase inhibitors selective for erbB-2 were reported.
Therefore, it would be greatly beneficial if new therapies could be designed based on identified existing compounds, rationally modified compounds and/or de novo designed compounds which are active as erbB-2 kinase inhibitors. In particular, it would be helpful if therapies based on compounds having improved selectivity, solubility and stability could be obtained.